1. Field of Invention
The present invention relates to computer technology and, more specifically, to a solid state disk drive emulation device with improved error correction.
2. Related Art and Other Considerations
Computer systems commonly employ magnetic hard disk drives for mass storage. Magnetic disk drives, however, are bulky, require sophisticated read/write electronics, and use high precision moving mechanical parts subject to reliability problems. Further, disk drives require a significant amount of power and are, therefore, less desirable for use in portable computers.
The dramatic increase in storage density of nonvolatile solid state memory devices, such as Flash EEproms, allows solid state disk drives to emulate the magnetic disk drives for mass storage in computer systems. Similar to magnetic disk drives, nonvolatile solid state disk drives retain data even after the power is shut down. Flash memory is also inexpensive and more reliable relative to magnetic disk drives because there are no moving parts. Further, solid state disk drives use less sophisticated read/write electronics and consume less power. Commonly, a PCMCIA card incorporates the Flash EEproms into a compact and portable device. Therefore, solid state disk drives are more suited for particular applications such as portable computers.
Obviously, there are drawbacks to using solid state disk which prevent magnetic disk drives from becoming obsolete. For instance, storing extremely large amounts of data (several hundred megabytes) is far less expensive in magnetic disk drives than in solid state drives. Also, solid state memory, such as Flash EEproms, have a limited lifetime and can be unreliable after a certain number of erase/write cycles (typically 1,000 to 10,000 cycles). A further drawback is that errors arising from defective memory cells, or discharged memory cells, can render the solid state disk drive less reliable.
One method for overcoming errors caused by defective memory cells is disclosed in U.S. Pat. No. 5,297,148 issued to Harari et al. The Harari patent discloses a method for replacing a defective memory cell with a redundant cell. A defect pointer which connects the address of the defective cell to the substitute cell is stored in a defect map. Every time the defective cell is accessed, its bad data is replaced by the good data from the substitute cell. This method, however, is undesirable in that the defect mapping is performed only during manufacturing and does not compensate for cells that fail during normal operation. Further, the rate of failures in Flash EEprom devices increases with increasing erase/write cycling.
Conventional error correcting (ECC) for magnetic disk drives is typically employed to compensate for errors caused by cells that become defective over time and for soft errors caused by discharged cells. The magnetic disk ECC systems, however, are optimized for errors that are associated with magnetic recording. Errors in magnetic recording are typically burst errors (several sequential bits) of varying length caused by a defect in the magnetic media. Conventional ECC systems employ complex error correcting algorithms to correct these errors. Consequently, implementing the algorithms requires complex circuitry that consumes more power and expense. Further, the significant amount of latency inherent in the complex algorithms for detecting and correcting errors associated with magnetic recording can slow the overall access time of the device.
The errors associated with solid state disk drives are significantly different than magnetic disk drives. Normally, solid state disk drives exhibit a one or two bit error per sector. The errors are commonly caused by a defective cell (hard error) or a cell that becomes discharged over time (soft error). It is inefficient to apply the complex error correcting algorithms for correcting errors associated with magnetic media in order to correct the one or two bit errors per sector commonly associated with Flash memory.
It is, therefore, a general object of the present invention to provide an improved Flash EEprom solid state disk drive that can serve as a non-volatile storage device in a computer system. A further object is to minimize the complexity and latency in correcting the types of one or two bit errors per sector commonly associated with Flash EEprom memory used as the storage element in the solid state disk drive.